This invention relates to a device for measuring a luminance of a fluorescent screen of a cathode ray tube.
Luminance measurement devices for a cathode ray tube (referred to as CRT hereinafter) have been so designed as to receive light from the fluorescent screen in synchronism with a vertical synchronizing signal to the CRT in order to assure an accurate measurement. Conventional luminance measurement devices have a fixed integral time of 100 milliseconds during which electric energy obtained by a photoelectric conversion element is integrated and then converted to a digital signal because synchronizing signals of many television CRTs have a fixed frequency of 50 Hz or 60 Hz.
These conventional luminance measurement devices provide an accurate measurement as a result of the long integral time. Conversely, they are difficult to perform fast signal processing and fast luminance measurement.
Also, CRTs for graphic display have various frequencies of vertical synchronizing signal which are from 40 Hz to 80 Hz. In such CRTs, the conventional luminance measurement devices having a fixed integral time cannot provide accurate measurements for all the CRTs having different frequencies of vertical synchronizing signals.